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I have a simple problem yet calling for a smarter solution than the one I came up with. I have a circuit that can receive power from different sources.

More specifically, the GND line is shared, but the VCC can come from different pins. Only one of these pins can happen to be connected at a time.

My requirement is that not only I need to be able to gather power from any of them, but I also need:

  • to sense which pin the power is coming from
  • to retain full voltage

I'm attaching a schematic with an inadequate solution, just to show what the situation is:

enter image description here

Thanks to the diodes I'm able to let power flow to the circuitry from any entry pin, but also preserve the ability to sense which is the only one that is powered (the other ones being discharged by the pull-down resistors).

The problem is that diodes cause a voltage drop that I cannot admit, even more so because such drop is going to vary according to the largely variable load of the rest of the circuitry. I thought of replacing standard diodes with ICs behaving as ideal diodes, but the cheapest ones that I can find still appear to be too expensive to be adopted in large quantities. Can anyone maybe suggest a different approach?

ocrdu
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lesath82
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    How much voltage drop can you tolerate? What's the maximum load from the rest of the circuit? You could use Schottky diodes for a lower drop. – Finbarr Jul 22 '22 at 08:03
  • Related to this [earlier closed question that the OP was advised to modify](https://electronics.stackexchange.com/questions/628279/ideal-diode-alternative). – Andy aka Jul 22 '22 at 08:21
  • @Finbarr Virtually, zero drop: I need to control precisely the voltage reaching the final stages. I *might* accept a drop from, say, my 3.0V to 2.65V *if* I was sure to have it always the same, but since the load may vary from almost 0 to many tens of mA, with Schottkys I'd have this 0.35V variation which is already too much. – lesath82 Jul 22 '22 at 08:54
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    Using mosfet + npn bjt drivers? – Antonio51 Jul 22 '22 at 13:03
  • @Antonio51 You are suggesting a scheme like this, right? https://electronics.stackexchange.com/questions/223935/understanding-an-ideal-diode-made-from-a-p-channel-mosfet-and-pnp-transistors – lesath82 Jul 22 '22 at 13:11
  • Interesting, but not exactly ... I will test my suggestion ... to check if it works. – Antonio51 Jul 22 '22 at 13:47
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    You could use regular diodes and a buck-boost converter to guarantee an exact supply voltage under all circumstances. – Finbarr Jul 22 '22 at 14:13
  • @Finbarr Thank you for the suggestion... in fact, I already considered the idea, and I estimated it has no hope of being cheaper than using ideal diodes instead of regular ones. Am I wrong? – lesath82 Jul 22 '22 at 14:42
  • @Antonio51 I'm looking forward to knowing your solution! (I was wondering if you had written NPN by mistake, since the ideal diode equivalent circuit I knew requires PNP) – lesath82 Jul 22 '22 at 14:46
  • We don't get into details on pricing on SE sites as there are so many factors involved and the answer would be out of date almost immediately. But if you want a specific set of features or level of performance you usually need to pay for it. – Finbarr Jul 22 '22 at 14:52
  • Tried with NPN + MOSFET P switch ... Failed. Tried with NPN + PNP switch ... seems ok. – Antonio51 Jul 22 '22 at 15:15
  • NB: until 30 mA, lost voltage = 0.2 V with BC547c and BC557. – Antonio51 Jul 22 '22 at 16:05
  • @lesath82 **many tens of mA** What are exactly your load specs? 1mA, 10 mA, 50 mA, 250 mA? – Antonio51 Jul 22 '22 at 16:11
  • I'm not exactly sure right now, but I expect almost 100mA, so I'd love to have everything appropriate for the range 0~150mA – lesath82 Jul 23 '22 at 09:38
  • @Finbarr Of course, everything comes for a price, and no doubt your suggestion would do the trick, I'm only supposing that my working solution with IC ideal diodes would still be cheaper than buck-boosts, so I'd stick to them (or drop the project) unless I'm suggested something even cheaper. Anyway, thank you! – lesath82 Jul 23 '22 at 09:55

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Ok. Voltages are not defined when I did this. -> some update needed.

Here is an example of a circuit that could be used. Enjoy it.
Interactive DC analysis. Microcap v12 simulator.
Don't forget that same "labels" are interconnected (Vload).

Tested under 2 configurations. The "switching" is made by V1 or V2 which "must" (?) be "exclusive".

All inhibited.

enter image description here

One circuit allowed to function.

enter image description here

Antonio51
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  • The OP said that they can not tolerate **any** voltage drop, particularly a voltage drop that varies with the load. It looks like you are dropping over 100mV. – Elliot Alderson Jul 22 '22 at 15:40
  • Right. But no specs. Probably should use mechanical relays, because active electronics always should fail. – Antonio51 Jul 22 '22 at 16:13
  • @Antonio51 Wow, this is very interesting! Thank you for the MC12 files, I'll use them to deepen my understanding of your suggestion, maybe it can even be optimized a little to reduce the voltage loss (which is actually still high). Moreover I will try to simulate the "handmade" ideal diode whose inspiration came from your previous comment, for which I'm grateful! – lesath82 Jul 23 '22 at 09:34
  • Yes, voltage loss is "high". Must search for a BJT that could be ok with very little Vce sat... MOSFET P did not work, surely because of the internal diode between Drain and source ... Search for a MOSFET P that does not have it ... and low logic voltage (3.3V) compatible. – Antonio51 Jul 23 '22 at 11:58